Determining the acoustic properties of downhole fluids may be desirable for several types of downhole evaluation. Such properties may be used in characterizing the fluid itself, or for use in methods for evaluating the formation, the borehole, the casing, the cement, or for previous or ongoing operations in the borehole including exploration, development, or production.
As one example, it is known to conduct acoustic inspection of a casing cemented in a borehole to determine specific properties related to the casing and surrounding materials. For example, the bond between the cement and the casing may be evaluated, or the strength of the cement behind the casing or the casing thickness may be estimated, using measurements of reflected acoustic waves, which may be generally referred to as casing cement bond logging. Physical properties of fluids vary at different depths of a well. Thus, for many of these techniques, it is desirable that variations in the fluid filling the borehole (e.g., drilling fluid) be compensated for, because conventional processing is highly sensitive to the properties of the fluid. So as one example, localized estimation of downhole fluid impedance may be desirable to enable accurate interpretation of downhole casing inspection measurements.
Thus, various techniques are currently employed to determine parameters of the fluid affecting acoustic measurements, such as acoustic impedance and sound velocity in order to interpret the acoustic reflection data. Traditionally, time of flight of the acoustic signals has been used to determine sound velocity, and additional measurements may be used to estimate at least one of acoustic impedance and density of the fluid.